1. Field of the Invention
This invention relates to an electron beam exposure method and, in particular, to an exposure method which provides a proximity effect correction when forming high precision patterns.
2. Description of the Related Art
With the recent trend toward very dense semiconductor devices, it has become necessary to correct for the so-called proximity effect when fabricating patterns with very narrow line widths by electron beam exposure. The correction is intended to produce an effective exposure or stored energy per unit area that is uniform in each pattern. In this regard, the following method has been proposed in IBM J. RES. DEVELOP. Vol. 24, No. 5, Sept. 1980.
For instance, when as shown in FIG. 5, a large-area pattern 11 is in the proximity of a small-area pattern 12, sampling points 21 to 37 are established for respectively sections each having a predetermined area. Inn the large-area pattern 11, the sample points 22 to 37 are located around the periphery thereof. Next, initial exposure values are respectively determined for the patterns 11 and 12. The respective stored energies at the sampling points 21 to 37 when exposed in accordance with these exposure values are then evaluated. Afterwards, sampling points of the same evaluation value are gathered together, thus dividing the pattern 11 into division patterns 41 to 48 as shown in FIG. 6.
Afterwards, respective proper exposures to be allocated to the pattern 12 and the division patterns 41 to 48 are calculated. The calculation is performed such that the proper exposures thus calculated will make the respective stored energies at the sampling points 21 to 37 equal to each other. The respective stored energies at the sampling points 21 to 37 when exposed in accordance with these proper exposures are then evaluated. If a sampling point of a different evaluation value exists in a division pattern, it is decided that the section including that sampling point does not belong to this division pattern, with the result that this division pattern is further divided into a number of division patterns including the above-mentioned section as one of them. For instance, if, of the four sampling points 34 to 37 in the division pattern 48, only the sampling point 36 exhibits a different evaluation value, this division pattern 48 is further divided into division patterns 49 to 51, as shown in FIG. 7.
Afterwards, the proper exposures to be respectively allocated to the pattern 12, the division patterns 41 to 47, and 49 to 51 are calculated. The calculation is performed so that the proper exposures obtained will make the respective stored energies at the sampling points 21 to 37 equal to each other.
In this way, the evaluation of stored energies, the division of patterns, and the calculation of proper exposures are repeated until all the sampling points 21 to 37 exhibit the same stored energy. Thus, proximity correction when forming patterns including very narrow line widths can be effected.
The problem with the above-described prior-art method is that the operations of evaluating stored energies, dividing patterns, and calculating proper exposures, have to be repeated many times before all the sampling points 21 to 37 exhibit the same stored energy, As a result, a lot of time has to be spent before the proximity correction is completed.